1. Field of the Invention
The present invention relates to a method of making graded index nitrogen-doped glass blanks from which optical devices such as waveguide filaments can be drawn.
Optical waveguides, which are the most promising medium for use in optical communication systems operating in the visible or near visible spectra, normally consist of an optical filament having a transparent core surrounded by a transparent cladding material having a refractive index lower than that of the core. The bandwidth of such filaments can be increased by forming the core thereof in such a manner that the refractive index is greatest along the longitudinal axis thereof and gradually decreases to a lower value at the edge thereof. Such gradient refractive index cores are often formed of a first material of relatively low refractive index such as silica to which there is added a dopant material to increase the refractive index, the concentration of dopant material being greatest at the fiber axis and gradually decreasing with increasing radius to provide the desired refractive index gradient.
2. The Prior Art
U.S. Pat. No. 3,999,835 issued Dec. 28, 1976 to G. R. Newns et al. describes an optical waveguide filament having a core region comprising silica doped with Si.sub.3 N.sub.4. That patent teaches that the silicon nitride doped silica glass can be formed in a boule by passing a mixture of gaseous compounds containing silicon and nitrogen through an induction coupled plasma discharge. To obtain an index gradient in such a nitrogen doped vitreous silica boule, the exterior thereof is oxidized by heating to a temperature of 800.degree. C. or above for an extended period of time. Newns et al. state that this heat treatment may be carried out before the boule is drawn, during the drawing operation, or the drawn filament may be heat treated. Diffusion controlled oxidation of nitrogen-containing glass does not provide adequate control of the index gradient to the extent necessary for manufacturing high bandwidth optical waveguides. The Newns et al. patent also teaches that silica-silicon nitride glasses can be deposited within a silica tube, the refractive index of the core being varied by varying the percentage of silicon nitride in the glass. This method has been found to be disadvantageous in that it results in the deposition of little, if any, nitrogen in the resultant preform.
Heretofore, nitrogen has been uniformly incorporated into a glass body by treating a glass body in the porous condition by subjecting it to a temperature between 900.degree.-1050.degree. C. in an atmosphere consisting essentially of ammonia gas, and its disassociation products, nitrogen and hydrogen, and thereafter heating and consolidating the porous glass body. Two methods of incorporating nitrogen into a porous glass body are taught in U.S. Pat. Nos. 3,113,008 and 3,149,946 issued to T. H. Elmer. The methods described in the Elmer patents result in a substantially uniform concentration of nitrogen throughout the resultant glass body. Such a uniformly doped preform is not useful, without further modification of nitrogen concentration, in the formation of high bandwidth graded index optical waveguide filaments.